The high energy forming techniques of the type under discussion use high explosives to form metal. These techniques normally use water or some other suitable fluid as a transfer medium for the mechanical energy produced by the explosives. It has been found that liquids transmit the mechanical energy generated more efficiently than air. Normally the process happens in an open tank. The charge of high explosive detonates in the water a short distance from the sheet of metal to be formed. The explosion causes pressure waves to transmit momentum to the metal and force it against the surface of a hollow die by plastic deformation.
The detonation wave that passes through the exploding charge interacts with the water in two ways. First, it creates in a liquid a shock wave that strikes the metal. The detonation wave also forms a bubble of compressed gas in the water. The bubble expands and contracts repeatedly as it reflects off the surface of the workpiece and sides of the tank before venting into the air. Though the peak pressure produced by the oscillating bubble is perhaps only 10 to 20 % of the peak shock wave, the bubble's contribution to forming the metal is also significant. The gas pressure lasts longer than the initial shock wave.
Many different materials are used in the dies for explosive forming. Inexpensive dies of zinc alloys, epoxy resin, or even hard wood are tough enough to make small numbers of products with limited accuracy. Plaster is used for dies to be used only once. Using reinforced concrete dies, usually resin coated, is an efficient way to make large parts in small numbers. If a manufacturer wishes to make a lot of parts, then the dies must be made of ductile iron or special steels which can be reused many times.
The advantage of these techniques is that large complex or compound curved shapes can be formed without the need for heavy presses and the very expensive conventional metal dies.
These known techniques generally require a vacuum to be applied between the surface and the sheet metal prior to discharge of the explosive to remove the air from the space that the metal will take up. If this is not done, the speed with which the plastic deformation of the sheet metal takes place is so fast as to cause a compressed air bubble to form, resulting in the distortion of the finished sheet metal and prevention of it flowing into the desired shape of the female die. The application of such a vacuum is simple when molding small shapes. However, when large complex shapes are to be produced in a relatively rough mold it is difficult to produce the appropriate vacuum required because of the need to obtain a seal between the workpiece and the surface. This process also adds costs to the process. While the terms "die" and "mold" have been used interchangeably hereinbefore, hereinafter the term "mold" will be used to mean either a die or a mold.